Not all skylights open--some are intended to let light in but not air. Other styles of skylight may have a small drop-down ventilation panel or screen that provides ventilation without opening the skylight. Many skylights, however, are designed to open. Some skylights will have a pole, chain or crank set at a low level so that they can be manually opened without having to climb on to a ladder or step stool.
Stand on a stable surface, like a step stool or a small ladder, to reach the skylight if necessary.
Locate the latch on your skylight. This is usually be a small lever that you flip to unlock the skylight. If there's no latch, your skylight won't lock shut, so you don't need to worry about unlocking it.
Press gently up on the skylight. If it doesn't move, it might be locked.
Flip the latch lever to unlock the skylight.
Press up on the skylight again. It should open upwards. If it doesn't, check to make sure that the latch is flipped all the way to the open position.
Latch the skylight open. Some skylights have a latch that will engage automatically, while others may have a small pole, not unlike the support for holding the hood of your car open, that you must slide into place.